Secretary of State John F. Kerry, left, talks with Yukiya Amano, director general of the International Atomic Energy Agency, during a meeting in Vienna on June 29. (Carlos Barria/AP)

While negotiators are still uncertain they can strike a deal on Iran’s nuclear program, a senior administration official said Monday it is “absurd” to believe that negotiators will give in to Iran’s demands.

Responding to critics who fear that the United States may be making too many concessions, the official said Washington is sticking to principles outlined in a framework agreement announced April 2 in Lausanne, Switzerland.

“Why would we put ourselves through this, why would our teams be here for as long as they have been, why would we be spending the hours doing this in the way we are if we were to just say, ‘Whatever you want, you got’?” the official said, speaking on the condition of anonymity to discuss the closed-door talks. “This is a very tough negotiation.”

The talks to curb Iran’s nuclear capabilities in exchange for sanctions relief simmered along Monday, as officials waited for Iranian Foreign Minister Mohammad Javad Zarif to complete a day-long trip to Tehran. He is expected to return Tuesday.

Secretary of State John F. Kerry met with Yukiya Amano, director general of the International Atomic Energy Agency, for the second day in a row. The negotiators are working closely with the IAEA, which is based in Vienna, because it would be responsible for sending inspectors to monitor Iran’s compliance with its commitments under a deal.

Officials say the talks, a culmination of 12 years of negotiations that gained momentum in 2013, will stretch beyond the Tuesday deadline but for no more than a few days.

“We are still very focused on concluding a comprehensive agreement in this negotiating round,” the administration official said. “And no one is talking about a long-term extension. No one.”

The key outstanding issues are the pace at which sanctions would be lifted and how much access the IAEA inspectors would have.

The official said negotiators are still tackling the timetable for sanctions relief but the IAEA would have to verify Iran’s actions first.

“Everybody will get ready, everybody will take their steps,” the official said. “And when the IAEA has verified it, virtually simultaneously, whatever commitments we make about the first phase of lifting of sanctions will occur.”

The official also said the negotiators from Iran and the six nations trying to work out a deal with it, known as the P5+1, are mindful that their decisions will be felt around the world.

“This is staggeringly consequential for everybody,” the official said. “Everybody who’s involved in this negotiation understands and quite frankly feels the burden of the responsibility of what we’re doing.”